The reconstruction of obstructed vital body organs to restore their function is a common and significant surgical task. In order to accomplish this task, surgeons have to perform extensive, traumatic operations with considerable morbidity and significant risk of severe complications and death. Therefore, it is desirable that alternative procedures be developed which allow the achievement of the same purpose with less risk and minimal surgical intervention.
With the advent of interventional radiology, researchers have developed techniques which effect the clearing of obstructions in many body conduits with little, if any, surgical intervention. The present invention relates to an improved nitinol spiral stent for use as an endoprosthesis for reinforcing these body conduits so as to maintain their patency and function. The invention has been demonstrated to have particular utility for repairing narrowed, weakened, distorted, distended or otherwise deformed tubular body conduits, such as those in the vascular, biliary, genitourinary, gastrointestinal and respiratory systems. The invention includes an improved delivery system for placement of the stent.
Previous devices for performing this function have not been designed for easy, controlled placement. Because the prior art stents were not designed to cooperate with specific placement devices and the placement devices were not designed to restrain the stent until properly located, correct placement was difficult to control. Also, prior nitinol stent systems required injection of cold saline, hot saline or both to prevent, control or encourage the transformation of the stent to its pre-set coil shape. This complicated the procedure and subjected the patient's body to unnecessary stress caused by the injection of the hot and cold fluids.
It is known in the art to utilize stents to reinforce tubular body conduits. Dotter (Investigative Radiology, Sep.-Oct. 1969, pp. 329-332) reported on unsuccessful experimental implantation of impervious plastic tube grafts and silicone coated stainless steel springs. In the same paper he also reported the successful (2.25-2.5 year patency) placement of coil springs made from standard No. 5 stainless steel wire. These coil springs were transported to the body site by a mandril with the coil tightly wrapped on its outer surface.
Additionally, it is known to utilize shape memory materials to fabricate these stents. The shape-memory effect was presented by A. B. Greininger of Harvard and V. G. Mooradian of MIT as early as 1938. Since then, several investigators have shown that there are several different types of alloys which demonstrate this property. More recently, U.S. Pat. No. 4,950,258 describes a molded coil produced from shape-memory polymers of lactide homopolymers, glycolide homopolymers, or copolymers of lactide and glycolide. However, this phenomena was not widely known until 1962 when Buehler announced the development of shape-memory materials prepared from titanium-nickel (TiNi) alloys, usually referred to as nitinol. Shape-memory alloys have the unusual property of mechanical "memory." These materials can be formed into a first predetermined shape above a transition temperature range (TTR), the TTR being dependent on the particular ratio of metals in the alloy. Below the TTR the alloy is highly ductile and may be plastically deformed into a second desired shape. Upon reheating above the TTR the alloy returns to its first pre-set form.
The general properties of these TiNi shape-memory materials are related in an article by W. J. Buehler, et. al., Wire Journal, June, 1969, pp. 41-49, and more extensively discussed in an article by McDonald Schetky (Scientific American, November, 1979, pp. 74-82). Schetky reviews several promising medical applications of the TiNi alloys including mounting means for orthopaedic implants, bone fracture binding systems, and implantable blood clot filters which can be inserted in a blood vessel through a catheter. These blood clot filters are further detailed in U.S. Pat. No. 4,425,908 to Simon, U.S. Pat. 4,494,531 to Gianturco, and Simon, M., et. al., Radiology, October, 1977, pp. 89-94 .
The Simon device uses a nitinol material with a TTR of 40-50.degree. F. The alloy, which can be formed into various mesh or filter designs, is reformed into a straight wire in ice-cooled water. A stud attached to the rear of the straightened filter wire is engaged in a notch on the guide wire which is then placed in the bore of a Teflon catheter for implantation. Under fluoroscopic control the catheter is advanced to the desired placement location within the blood vessel, and the guide wire and free end of the nitinol wire are extruded from the catheter. As the straightened nitinol wire becomes exposed to the temperature of the blood, it reforms into its pre-set filter shape locking it into the blood vessel and, as the notch on the guide wire with inserted stud on the straightened wire are extruded from the placement catheter, the wire automatically becomes disengaged from the placement device.
Other medical applications of shape-memory alloys in medicine are a prosthetic pump (U.S. Pat. No. 3,827,426), an apparatus for percutaneous catheterization (U.S. Pat. No. 4,411,655), an intrauterine contraceptive device (U.S. Pat. No. 3,620,212), a ring to hold a sewing cuff to an artificial heart valve (U.S. Pat. No. 4,233,690), a catheter or cannula (U.S. Pat. No. 3,890,977), marrow nails (U.S. Pat. No. 4,170,990), and dental arch wires (U.S. Pat. No. 4,037,324). Also, U.S. Pat. No. 4,665,906 addresses the use of stress-induced martensite alloys for the formation of several different medical devices.
Most relevant to the present invention are the products described by Dotter ("Transluminal Expandable Nitinol Coil Stent Grafting: Preliminary Report," Radiology, 147, April, 1983, pp. 259-260 and U.S. Pat. No. 4,503,569), Cragg, et. al., ("Nonsurgical Placement of Arterial Endoprosthesis: A New Technique Using Nitinol Wire," Radiology, 147, April, 1983, pp. 261-263), Alfidi, et. al., U.S. Pat. No. 3,868,956 and Balko (Transfemoral Placement of Intraluminal Polyurethane Prosthesis for Abdominal Aortic Aneurysm, Journal of Surgical Research, 40, April, 1986, pp. 305-309; and U.S. Pat. No. 4,512,338). The Dotter article discloses the use of a nitinol material having a transition temperature of 130.degree.-140.degree. F., thus, requiring an injection of hot saline to cause the material to reform to its coil shape. Despite the specific requirement for hot saline, the Dotter patent claims a nitinol material with a transition temperature in a range at or above normal body temperature. Because the Dotter patent does not incorporate any means to restrain the coil during insertion, the placement procedure requires an initial injection of cold saline to insure the coil does not reform during insertion followed by a second injection of hot saline to encourage reformation. Cragg utilizes a threaded adapter on one end of the coil to hold it during placement. Alfidi discloses an appliance expansible by the application of electrical heating. Balko discloses a nitinol wire with a transition temperature below body temperature. In order to prevent the stent from changing shape while being positioned in the body it is enclosed in an insulating sheath.
G. Mednik, in an article entitled "Roentgeno-endoesophageal Prosthesing with Nitinol Spiral," published in the Russian journal Surgery, pp. 87-94 (1989), along with other clinicians, reports on the use of nitinol stents to clear obstruction in the esophagus of cancer patients. The placement procedure described therein restrains only one end of the stent, leaving it free to deform as it is heated by the patient's body temperature.
All of the stents described in the prior art patents and articles suffer from the same deficiencies. The stents and delivery systems are not designed to work together to allow accurate and controlled placement of the device.. while the shape-memory property of the nitinol materials is utilized, maximum advantage of this property cannot be realized because the design of the prior art delivery systems does not adequately control the transformation of the stent from its cold temperature form to the pre-set form attained above its TTR. A potential problem of nitinol alloys with a high TTR (i.e., 130.degree.-140.degree. F.) is they could be easily deformed in situ at body temperature. Unrestrained nitinol stents having a TTR below body temperature require rapid placement as they will start transforming to their original shapes as soon as they are introduced into the body or blood stream.
Thus, there is a need for a stent and delivery system which overcomes these problems.